The development of a novel technique for small ring specimen tensile testing

Abstract

The wide scale use of small specimens in routine testing programs could significantly reduce material resource requirements (factors of 10 are easily achievable). This is a major benefit to situations where there is not enough material to manufacture conventional, full-size specimens, such as first-stage gas turbine blade roots. However, limitations exist due to concerns over size effects, manufacturing difficulties, uncertainties related to the application of representative loading conditions and complex interpretation procedures of non-standard data. Due to these limitations, small specimen testing techniques have been mostly applied in ranking exercises and to determine approximate or simple material parameters such as Young's modulus, minimum creep strain rate and fracture toughness. The small ring method is a novel, high sensitivity small specimen technique for creep testing that has been extended in the present work to the determination of tensile material properties. The main advantages of the small ring specimen are that it is self-aligning and has a large equivalent gauge length in comparison to other small specimens, resulting in much higher testing sensitivity. In the present work, this specimen type mimics conventional, full-size, monotonic testing, allowing for observations of elastic and plastic material response to be made. Wrought aluminium alloy 7175-T7153 small rings were tested at room temperature at 5 different loading (displacement) rates and the results compared to conventional, full-size, monotonic specimen equivalents. Finite element analysis was conducted in order to evaluate the equivalent gauge section and equivalent gauge length in the small ring specimen (which varied between circa 0.35-1.4mm 2 and 25-45mm, respectively) to facilitate these comparisons. An analytical solution has also been derived in order to validate the finite element analysis.

On the use of small ring testing for the characterisation of elastic and yield material property variation in additively manufactured materials

“…Two dimensional plane stress approximations are usually cheaper than three dimensional equivalents (assuming similar levels of discretisation), thereby reducing the time taken per iteration. Previous work [31] has verified that, for the modest displacements considered in the present work, the difference between plane stress and full three dimensional elements representations is negligible. For this reason a quadratic shape function variant of the former is adopted here.…”

“…When attempting to convert force/displacement type data to the equivalent uniaxial stress/strain data it is vital that both of these sources of non-linearity are accounted for if underlying material response is to be extracted correctly. It is often erroneously remarked that the onset of non-linearity in small ring force/displacement data is due to yielding, however it is easily shown (see Kazakeviciute et al [31], for example) that plastic strain begins to accumulate long before this point. A reasonable approximation of non-linear stiffness may be achieved with a semi-analytical approach (such as the one developed in section 3.3), however accurate analysis of small ring test results is most readily achieved using finite element analysis.…”

“…By recalling the specimen dimensions used in the present work two alternative continuum FE formulations may be judged as appropriate, namely two dimensional plane stress and three dimensional ("brick") elements. It is clear that a full three dimensional representation of the small ring specimen is closer to reality as through thickness effects (such as anticlastic curvature, see Kazakeviciute et al [31]) can be approximated. This additional fidelity comes at a computational cost which can quickly become significant in iterative procedures such as the inverse one implemented in the present work.…”

“…Small rings are favoured over miniature tensile specimens in the present work due to the ease of testing, large equivalent gauge length, and self aligning nature. The more complex stress state induced in small ring specimens does complicate data interpretation, however it has been shown in the authors' previous work [31] that the specimen is sensitive to the material and the the inverse method developed in the present work provides an approach to extract constitutive parameters. The work of Lancaster and co-workers also deserves particular attention here [34,35], where small punch test based methodologies were developed for characterising tensile and fatigue properties.…”

“…Small specimens can be used to acquire properties related to a range of physical phenomenon such as tensile behaviour [23,24], fatigue [25], fracture [26], crack propagation [27], and creep [28,29]. A number of small specimen types have been developed [29], including the small ring specimen, initially devised in 2009 by Hyde and Sun [30] for the evaluation of creep properties and since developed by Kazakeviciute et al [31] for the evaluation of tensile properties of materials. This small specimen type has several advantages over alternative methods, such as having a large equivalent gauge length (approximately 50 mm, providing high test sensitivity), being self-aligning in nature, demonstrably insensitive friction conditions at the "grips" (pins), and simple to manufacture.…”